Radiused End Mills

[tornitore45]

I got some carbide end mill at a flea market. They seems OK, I suppose the low price is due to not being on size, but they have a rounded edge. Not talking about ball end, just a 1/16 or 1/32 radius over a 1/2" tool.

I assume they are used when a fillet is desired at an inside corner.

My question: Is there a penalty in using them for general purpose metal removing, basic sizing?

[earlgo]

I have some radiused end mills that were bought from a used tool dealer. My bet is that the sharp corners were chipped and this is an easier fix than cutting back the end and re-sharpening the center cutting feature. I use them for general use, and yes they make a nice corner radius which is critical if the parts are highly stressed.
In the design world this sharp corner is considered a stress concentration and reduces the allowable stress by as much as 1/2 to 1/4 depending on the application.

--earlgo

[GlennW]

The first thing to go (chip) on a carbide end mill is the sharp corner resulting in a lousy finish. Starting with a small raduis can extend the useful life.

I have some with a .005" radius for CNC work for that reason. Leaves a nice finish as well.

The larger radius (or the smaller one) is used for stress relief on inside cormers as mentioned above.

[Jaxian]

I need to start being a bit faster. Always posting after Glenn. He is fast. That said.

What Glen said above. I just finished reading a entire section of the 'Treatise on Milling' by Cincinnati Machines chief engineer about why you want a radius on pretty much every cutter, how big it should be why it should be that size depending on the work you do..etc.

According to them unless a cut requires a hard sharp corner you should be using a cutter with a radius on the edge. For high production cuts, heavy use, or as Glenn said CNC speed if you don't leave some type of radius there you will torch/chip/wear the sharp corner in short order rendering the cutter useless for it's intended job.

What you don't want is cutter where it looks like someone just ground the edge to cover up a chipped flute. Although if you are buying stuff that was used in CNC machines they have a lot of 'special grind' cutters that are custom made to make some feature they can't do any other way. If you want pictures of normal radius edges, or funny cut edges. or ones that aren't prepped for use yet I have a couple hundred cutters so have a good representation of some really weird geometry. Some of which is very useful, some of which I think was just trying to salvage a cutter.

Btw, when buying used end mills always, always, always measure the diameter before using them. Chances are they were reground at some point and nothing is more frustrating than get a whole operation set up making a cut and realizing your 7/8"(.8750) marked cutter was really .833 and you have to move things and make another pass. I measure and mark mine whenever I get used ones. Time consuming but worth it. If you have a CNC this isn't an option, you must know the real diameter and OAL of the end mill or things can get ugly.

[Jerry_H]

That begs the question. Do all the end mills you buy from the various tool companies have a slightly rounded corner? Or are they all square? I can't say that I have examined any close enough to know.

Jerry

[Jaxian]

That begs the question. Do all the end mills you buy from the various tool companies have a slightly rounded corner? Or are they all square? I can't say that I have examined any close enough to know.

Jerry

Good question. I usually buy only used endmills from the local machinery place as they are a fraction of the cost of new ones and I can get good US made cutters for $5 instead of $60 or more. Usually that means at least one resharpen. The ones that are unused and untouched that I buy do often have razor sharp leading edges. But I have to say I don't think I see a single used one that was sharpened that doesn't have some radius on it. Even ones that are still the full diameter and have never been actually put to work have a reground tip. It would appear that companies that bought them would as a first step have them reground to suit whatever work they were doing.

That would at least explain why none of the new ones ready for use don't have a regrind. You can order number of flutes, lead angle, OAL, coating, material, and other parameters that you can't change then grind the end for your application. I am basing this on only about 300 end mills that I personally own, I don't have thousands like some guys so just what I have observed.

Here is a few examples. They are both unused but as you can really see on the one on the right the coating was ground off when they put a radius on it:

On a close up of the smaller one you can see it too has a radius ground in it:

On this one though it was right out of the box (literally, still has the cardboard dust from the tube on it). Not yet sent for prepping. Razor sharp edges. Insanely expensive mill new, doubt they would use it without putting a radius on it to extend it's life. Especially as it's a roughing mill. Hard to see but the little 1/2" one next to it also has a small radius ground into it.

But again this is just what I have observed from the ones to pass through my hands and talking to guys doing work on specific metals/jobs. They like to put their own regrinds on them. Actually I was told around here when I got a big lot from Boeing they actually had a company requirement that there be NO endmills used that had not been radiused to their spec. Something to do with stress cracks and stuff. I remember thinking that would suck to be the guy who has to sit there all day and regrind the tips of every new cutter to the Boeing spec. That would get really old I would guess.

[Harold_V]

On a close up of the smaller one you can see it too has a radius ground in it:

Not a radius. A chamfer. It, too, serves to prolong the cutting edge, and may have been ground because of a chipped tip, but not necessarily. A chamfer is quite good at expelling heat, unlike a sharp corner.

Addressing the condition of new end mills; unless a bull nose is specified, they come with sharp corners. It costs for the radius to be added, and some prefer it not be there.

It was common practice for the operator to apply a corner radius when it was required. That, of course, was long before CNC and a wide variety of insert tooling that is now commonly available.

Harold

[tornitore45]

Very interesting responses, what was supposed to be a possible drawback turned out to be a desirable feature often used in industry.
Makes me fell better about a couple end mills I mangled by grinding a chamfer after burning the very tip. They do work fine after all but I kept them for roughing duty.

[Jerry_H]

Thanks, Jaxian and Harold. I love this board as I am always learning something new and useful.

Jerry